Back in 1886 a multi-national agreement concerning copyright was signed in Berne, Switzerland. This agreement is known as the Berne Convention and applies to the signatory countries. The United States did not ratify the Berne Convention and become a signatory until 1989. That's right. It took the United States more than a hundred years to accept the agreement. The main provision of the Berne Convention, now enforceable in the United States, is that copyright protection becomes automatic and any requirement for formal notice is prohibited. This agreement applies to all of the Berne Convention signatories. See the list from the World Intellectual Property Organization.
This means that any "work" as defined by the U.S. Copyright Law becomes automatically covered without any notice. It is not necessary for the work to have the copyright symbol or any mention of copyright to be protected. The term of the copyright is determined by the country of origin.
So why are people able to republish photos and other content on the Internet? This is a really interesting question. You cannot assume that an image that is being "passed around" on the web is not legally subject to a claim of copyright. This is doubly the case if you find an image on someone's website. In fact, you must assume that the document is subject to a copyright claim unless there is specific information to the contrary.
In the United States, part of the issue of online content was addressed in another major copyright statute. In 1996 the United States passed the Digital Millennium Copyright Act implementing the provisions of two World Intellectual Property Organization treaties. The provisions of this statute are summarized by Wikipedia as follows:
The Digital Millennium Copyright Act (DMCA) is a United States copyright law that implements two 1996 treaties of the World Intellectual Property Organization(WIPO). It criminalizes production and dissemination of technology, devices, or services intended to circumvent measures (commonly known as digital rights management or DRM) that control access to copyrighted works. It also criminalizes the act of circumventing an access control, whether or not there is actual infringement of copyright itself. In addition, the DMCA heightens the penalties for copyright infringement on the Internet.A complete copy of the legislation can be found on the Copyright.gov website.
So what if I have an image and I do not care about copyright protection? How can I notify the world that they can use my image without worrying that I will send them a nasty letter?
There are various ways to legally release all or part of your copyright interest in any work. One of the most commonly used methods for works on the web is the use of the CreativeCommons.org. If you post a work on the web, you can retain your copyright, but at the same time, you can specify a less restrictive use policy. You can do this for your whole website or for individual documents. You will need to go to the CreativeCommons website and spend some time studying the different levels of license that you can give to those who use your work.
If you are trying to find public domain images or other works on the web, be sure to look carefully at the content. If the image appears on a webpage without any indication of copyright or license, it is protected under copyright and it would not be a very good idea to assume otherwise. In other words, you must have a positive assertion that a document, image or any other work has been released from its copyright either due to a specific license or the passage of time.